464 SUMMARY OF CURRENT RESEARCHES RELATING TO 



roots, with definite cortex. This is comparable with the case of 0. 

 cinnamomea, where roots often enter the pith through the branch gap 

 at a dichotomy. 4. The leaf -trace is monarch, and occasionally mesarch 

 at the base. It expands into a single endarch arch, which in the base of 

 the petiole is surrounded by a ring of sclerenchyma. On either side of 

 this, in the cortex of the stipular wing, is a large island of sclerenchyma, 

 and there is a patch of the same tissue inside each lateral bay of the 

 leaf -trace. 5. Osniundacere and Zygopteridete are not closely related, 

 differing so widely, as they do, in anatomy of leaf-trace and foliar 

 bundle. The leaf-trace of the Osmundaceas and Ophioglossacere is 

 typically monarch, and that of Zygopteridese diarch. The simple con- 

 dition of Glepsydropsis probably led rather to the diarch and triarch 

 modern ferns than to Osrnundacese. 6. There seem to have been both 

 protostelic and siphonostelic Osmundacese in ancient times, and there is 

 no convincing evidence that protostelic members, such as Thamnopteris, 

 have given rise to species with a pith, for transitional forms are un- 

 known. But there are transitions between primitive siphonostelic 

 types, such as Osmundites skidegatensis, and the most reduced modern 

 species. The xylem elements described in the "mixed pith" in 0. 

 Kolbei lire probably root-bundles like those of 0. Dunlopi. 7. The theory 

 of the origin of the Osmundaceae, which assumes that they have been 

 reduced from typical siphonostelic forms, has the advantage of explain- 

 ing very many structural facts in living and fossil members of the family 

 which remain unaccounted for on any other hypothesis. 



Ulodendroid Scar.*— D. M. S. Watson discusses the structure and 

 origin of the Ulodendroid scar. His summary is as follows. 1. In 

 Lepidodendroids small branches issuing laterally were sometimes shed 

 by an abscission layer, developed from a cambium which forms across 

 the base of the branch in all its living tissues, except that part of 

 the primary outer cortex which lies outside the secondary cortex. This 

 abscission layer becomes connected with the secondary cortex of the 

 main stem, and in old age entirely takes on the structure of the latter. 

 2. The Ulodendroid scar is such an abscission layer cutting off a branch 

 which was formerly attached to its whole area. 3. Halonia is essentially 

 similar in that it represents a stem from which lateral branches have 

 been cut off by an abscission layer. 4. The most practical distinction 

 between Halonia and Ulodendron lies in the arrangement of the branches, 

 but the scars themselves are quite different when well preserved. 5. 

 The well-preserved Halonial scar is divisible into two areas, of which 

 the inner represents the abscission layer, whilst the outer results from 

 the pressure of the long leaf-bases of the branch on those of the stem ; 

 consequently the Halonial condition can only occur in Lepidophloios. 

 6. In some cases the lateral branch of Halonia was not the peduncle of 

 a cone. 7. No specimen of Ulodendron showing a well-preserved leaf- 

 scar has ever been described, except in that type with oval scars and an 

 excentric umbilicus, which belongs to Bothrodendron. 8. Consequently 

 it is best to retain the generic name Ulodendron, as is done by most 



♦ Ann. of Bot., xxviii. (1914) pp. 481-98 (1 pi. and 2 figs.). 



